1. Technical Field
The present disclosure relates to a MEMS (microelectromechanical systems) device and to a corresponding micromechanical structure, with integrated compensation of thermo-mechanical stress (or strain). The following description will make particular reference, without this implying any loss of generality, to an inertial sensor device, such as a linear accelerometer.
2. Description of the Related Art
As is known, micro-machining techniques for semiconductor devices enable manufacturing of micromechanical structures within layers generally of semiconductor material, which have been deposited (for example, a layer of polycrystalline silicon) or grown (for example, an epitaxial layer) on sacrificial layers, which are removed via chemical etching.
For example, MEMS inertial sensor devices are known, including at least one micromechanical structure integrated in a die of semiconductor material and having mobile regions (the so-called “rotor regions”) suspended with respect to a substrate of the die, and fixed regions (the so-called “stator regions”) anchored and fixed with respect to the same substrate and in particular to a package of the MEMS device. The mobile regions are connected to anchorages fixed with respect to the substrate via interposition of elastic biasing elements (springs). In the presence of a quantity to be detected (for example, an acceleration), the mobile regions move by inertial effect with respect to the fixed regions, along one or more axes, which constitute the sensing axes of the sensor.
When a sensing capacitive principle is adopted, the mobile regions and the fixed regions are capacitively coupled to form sensing capacitors, the sensing capacitance of which has a value that is a function of the inertial movement of the mobile regions, and hence of the quantity to be detected.
The various regions forming the micromechanical structure may have different coefficients of thermal expansion, especially in the case where they undergo different dopings. Moreover, the material (typically plastic or ceramic) of which the package of the MEMS device housing the micromechanical structure is made has a different coefficient of thermal expansion with respect to the material of which the structure itself is made (generally, monocrystalline or polycrystalline silicon).
Stress deriving from the welding processes, or in general from thermal gradients generated during use of the MEMS device, are transferred from the package to the silicon die in which the micromechanical structure is provided.
Consequently, thermo-mechanical stress may arise in the die (for example, according to the phenomenon known as “die warpage”), and in particular strains may be transferred to the anchorages of the mobile regions and/or the fixed regions, even acting in a different and non-uniform way on the various anchorage points, which many undergo minor relative displacements with respect to one another. In general, tensile and compressive stresses may be generated, and the mutual position of the various parts of the structure may be modified.
Due to the above phenomenon, a variation of the sensing capacitance may thus be generated, even without the inertial quantity to be detected (for example, in the absence of an acceleration), with a resulting deviation (drift or offset) of the output value supplied by the MEMS device.
This entails alterations in the performance of the MEMS device, in particular measurement errors and drifts, which may even vary according to the production lot, and at times also among sensors belonging to one and the same production lot; these alterations may also vary in time.
In order to compensate for the aforesaid measurement drifts, a wide range of solutions has consequently been proposed.
In particular, some solutions generally envisage electronic compensation of the thermal drifts of measurement supplied by the micromechanical structure via the introduction of appropriate electronic components in the reading interface associated to the structure in the MEMS device, usually as an ASIC (Application-Specific Integrated Circuit).
For instance, a known solution envisages the use of a temperature sensor in the reading electronics associated to the micromechanical structure. Once the temperature is known, any drift of the system is compensated electronically by resorting to compensation curves previously obtained via appropriate calibration and/or simulation procedures.
Solutions of this kind are, however, burdensome in so far as they demand costly and delicate measurement procedures to obtain compensation curves that map accurately any thermal drifts of the sensors, and purposely devised compensation operations. In addition, the degree of precision that can be achieved is not in general altogether satisfactory and repeatable.
Other solutions that have been proposed hence envisage an integrated compensation of the thermo-mechanical stress by introduction of structural compensation elements in the same micromechanical structure.
For example, U.S. Pat. No. 7,646,582 discloses a MEMS device in which, in addition to a micromechanical sensing structure, a micromechanical compensation structure is present, altogether similar to the micromechanical sensing structure, and is designed to feel the same thermo-mechanical stress and to be insensitive to the inertial quantities to be detected (in particular, linear accelerations).
The above solution, although advantageously providing an effective integrated compensation, utilizes, however, a considerable use of resources in terms of area occupied in the die of semiconductor material, given that it envisages the presence of a compensation mass (altogether similar to the inertial sensing mass), with associated electrodes, elastic elements, and anchorages. This solution is consequently difficult to implement in the case where the reduction in size and costs constitute an important design specification (as, for example, in the case of portable applications).
A further known solution, disclosed in U.S. Pat. No. 7,520,171 envisages the presence of only some integrated compensation elements, without replicating the entire micromechanical sensing structure and hence without requiring the presence of a further mass. The particular solution described in the patent enables, however, just compensation of planar strains (i.e., ones acting in a horizontal plane of main extension of the structure), parallel to the substrate of the die, but does not enable compensation of strain components acting out of the horizontal plane (in particular, acting in a vertical direction, orthogonal to the horizontal plane).